This invention relates to a fluidized bed. Such a bed may be used for example as a heating medium (when it could be used, inter alia, for removing paint or plastics materials from metal components or for the incineration of refuse and sewage) or as heat treatment apparatus which will carry out heat treatment processes (for example carbo-nitriding, oxy-carbo-nitriding, carburizing, neutral annealing, neutral stress relieving and neutral hardening) on a component or components immersed in the fluidized bed. When the fluidized bed is to be used as heat treatment apparatus, it is known to provide a bed formed of refractory particles, and means for supplying a gas or gas/air mixture, which not only provides a sufficient velocity of gas or gas/air flow for fluidizing the bed but which also provides within the bed in use the desired atmosphere for carrying out the required heat treatment process. In addition heat will, in use, be applied to the bed for an interval or intervals sufficient to maintain the bed at the desired temperature while the heat treatment is being carried out. Such heat may be provided in a number of ways. One way is to provide, for fluidizing the bed, a combustible (e.g. stoichiometric) mixture of fuel gas and air, this mixture then being ignited at the upper bed surface; the flame front will then gradually penetrate to the bottom of the bed. Alternatively, the fluidizing mixture can be heated to the desired working bed temperature before it is admitted to the fluidized bed. As a further alternative, cool fluidizing gases can be admitted to the bottom of the bed and heat supplied either by electrically energised elements disposed within the bed or by heating units disposed on the exterior of the bed wall. As yet another alternative, cool fluidizing gases can be admitted to the bed and heat supplied by means of combustion at or close to the upper surface of the bed, such combustion comprising for example "submerged" combustion (where gaseous fuel is burnt just beneath the upper surface of the bed) or alternatively a kind of "fireball" combustion where gaseous fuel is burnt just over the upper surface of the bed. The present invention is more particularly concerned with the arrangements in which cool fluidizing gases are admitted to the bottom of the bed.
In all cases however it is of course desirable to ensure, for reasons of efficiency, that the heat exchange which occurs in use between the particles of the bed and the work which is to be heated shall be as great as possible and it is known that this situation occurs when the velocity of the fluidizing gases (the optimum fluidization velocity) through the bed is approximately 2.5 to 3.0 times the gas velocity (the minimum fluidization velocity) at which fluidization first occurs. Factors which affect fluidization include the temperature of the bed itself and the size and density of the particles which make up the bed. Thus, for example a fluidized bed at a temperature of approximately 1000.degree. C. will require a minimum gas fluidization velocity of about one tenth of the required minimum velocity at ambient temperature. Furthermore, it is necessary that the gas velocity should reach a certain minimum value before heat will effectively travel downwardly from a source of heat in the bed. Such minimum value is about three times the minimum fluidization velocity which in fact is compatible with the gas velocity that is required to obtain maximum heat transfer from the fluidized particles to work inserted therein, as previously mentioned. It is therefore possible to control the temperature in a fluidized bed by controlling the velocity of the fluidizing gas. To this end it is known to provide an adjustable flow control valve or an adjustable pressure regulating valve in a supply pipe which supplies the fluidizing gas to the bottom of the bed.